In a downlink of a mobile communications system, in some cases, a radio base station has one physical channel shared between the radio base station and mobile stations belonging to the radio base station. Hereinafter, the physical channel used in this case is referred to as “downlink shared channel”.
In the downlink shared channel, the radio base station controls the order, in which packets are transmitted to a plurality of mobile stations communicating with the base radio station, based on instantaneous radio quality between the radio base station and each of the mobile stations. This controlling improves a throughput, which the radio mobile station can provide, in other words, so-called system capacity.
Such controlling of the transmission order of packets by a radio base station is called “scheduling”. It has been known that, by applying such scheduling to packet transmission, communications capacity is increased, or communications quality is improved (for instance, refer to Non-patent Document 1)
It has been generally considered that requirements for transmission delays are not so strict for target packets in conventional scheduling.
Incidentally, with regard to standardization of the third generation mobile communications system, so-called IMT-2000, there are “3GPP/3GPP2 (Third-Generation Partnership Project/Third-Generation Partnership Project 2)” which have been organized by local standardization organizations and the like. Standardization specifications have been developed as “W-CDMA system” in the 3GPP, and as “cdma2000 system” in the 3GPP2.
In the 3GGP, standardization of “HSDPA (High Speed Downlink Packet Access)”, which is a high-speed downlink packet transmission system, has been proceeding. The proceeding is based on an assumption that, with a rapidly-increased use of the Internet in recent years, high-speed and high-capacity traffic would increase due to, for instance, the downloading from databases and Web sites especially in a downlink (for instance, refer to Non-patent Document 2).
From the same viewpoint as above, in the 3GPP2 as well, standardization has been proceeding by means of “1x-EV DO” which is a transmission system dedicated to downlink high-speed data (for instance, refer to Non-patent Document 3) “DO” in the “1x-EV DO” of the cdma 2000 system means “Date Only”.
In the HSDPA, for instance, a scheme for controlling a modulation scheme and an encoding rate of radio channels depending on the radio quality between each mobile station and a radio base station (this scheme is referred to as AMCS (Adaptive Modulation and Coding Scheme) in the HSDPA), and the scheduling which is operated in a cycle of a few milliseconds, are used in a combination. This combination makes it possible to improve a throughput of each mobile station, and a throughput of the entire system.
As a scheduling algorism in a radio base station, a “Round Robin Scheduler” is widely known. The Round Robin Scheduler controls the transmission order of packets waiting for transmission, by allocating the downlink shared channel sequentially to mobile stations (for instance, mobile stations #1→#2→#3 . . . ) which belong to the radio base station.
In addition, as scheduling algorisms in radio base stations, a “Proportional Fairness Scheduler” and a “MAX C/I (Maximum C/I) Scheduler” are known as well, in which the transmission order of packets waiting for transmission is controlled, based on an instantaneous transmission rate and an average transmission rate of packets to each mobile station.
The “Proportional Fairness Scheduling” is a scheduling algorism, with which transmission queues are assigned depending on change in instantaneous radio quality in a downlink of each mobile station while fairness among each mobile station is supported.
By referring to FIG. 9, the Proportional Fairness Scheduling will be briefly described below. FIG. 9 is a flow chart showing operations of the Proportional Fairness Scheduler installed in a radio mobile station.
In the Proportional Fairness Scheduling, an evaluation function of each mobile station belonging to the radio base station is calculated based on a measured instantaneous transmission rate of packets to each mobile station (or instantaneous radio quality between the radio base station and each mobile station) and an average transmission rate of packets to each mobile station (or average radio quality between the radio base station and each mobile station). Thereafter, a transmission queue is assigned to a mobile station which maximizes an evaluation function (in other words, packets are scheduled).
As shown in FIG. 9, in step S1001, the radio mobile station sets initial values as follows:    n=1 (n: a subscript of a mobile station)    Cmax=0 (Cmax: a maximum value of an evaluation function)    nmax=0 (nmax: a subscript of a mobile station which maximizes an evaluation function).
In step S1002, the radio base station measures elements necessary for calculating the evaluation function. Specifically, the radio base station measures an instantaneous transmission rate Rn of packets to each mobile station #n and an average transmission rate of packets to each mobile station #n:
 Rn.
In step S1003, the radio base station calculates an evaluation function Cn based on the following formula, using the value measured in step S1002.
  Cn  =      Rn                  R        _            ⁢                          ⁢      n      
In step S1004, the radio base station determines whether or not the evaluation function Cn calculated in step S1003 is larger than the maximum value Cmax of the evaluation function.
Since Cmax=0 in this case, the determination in step S1004 is YES. In step S1005, the radio base station sets the value of Cn calculated in Step S1003 at Cmax, and sets nmax=1.
Thereafter, in step S1006, the radio base station increments n by +1. In step S1007, the radio base station determines whether or not n is larger than N (the number of mobile stations communicating with the radio base station).
In a case where n is not larger than N, the operation repeats steps S1002 to S1006, and thus N values of the evaluation function Cn are found sequentially.
In step S1008, the radio base station selects a mobile station #nmax maximizing the evaluation function Cn, and assigns a transmission queue to the mobile station #nmax.
In this case, the numerator of the evaluation function Cn is an instantaneous transmission rate of packets to each mobile station (or instantaneous radio quality between the radio base station and each mobile station), and the denominator is an average transmission rate of packets to each mobile station (or average radio quality between the radio base station and each mobile station). For this reason, in a case where an instantaneous transmission rate is higher than an average transmission rate, there is a higher possibility that a transmission queue is assigned to a mobile station #n.
Accordingly, in the conventional Proportional Fairness Scheduling, a transmission queue is assigned to a mobile station in a case where an instantaneous transmission rate of the mobile station is higher than its average transmission rate, regardless of whether the average transmission rate of the mobile station is higher or lower. This provides improvement in both throughput and fairness, which result from user diversity gain.    [Non-patent Document 1] J. M. Holtzman, IEEE VTC2000 Spring    [Non-patent Document 2] 3GPP TR25. 848 v4. 0. 0    [Non-patent Document 3] 3GGP2 C. S0024 Rev. 1. 0. 0
The conventional Proportional Fairness Scheduling, however, has a problem that packets cannot be scheduled to each mobile station in consideration of QoS (Quality of Service) of various services including streaming services and video phone services, differences in capabilities among mobile stations, equalization of the scheduling opportunities, and the like.